Audit README evidence and dashboard links

[HanSun103]

What changed

• rewrote the main README around the verified Ridge, XGBoost, and temporal ensemble evidence
• moved and clarified the active-system architecture
• documented the exact OOS period, sentiment-control boundary, tactical-routing status, and backtest limitations
• fixed the public dashboard's Live Forward Monitor link to target index.html
• added validation and regression coverage for the public navigation contract
• updated the two-page dashboard implementation record

Why

The README previously mixed metrics from different experiments and overstated several implementation details. The public dashboard also linked to a nonexistent dashboard.html file after publication.

Impact

Readers can now trace the headline metrics to the July 27 Ridge-XGBoost experiment, and both dashboard pages navigate correctly when deployed from main.

Validation

• 221 tests passed
• changed-file Ruff checks passed
• public dashboard bundle validation passed
• metric-to-artifact audit passed
• all local README links resolved
• static-site relative navigation passed
• git diff --check passed